Stephen Blackheath Anthony Jones [Stephen Blackheath - Functional Reactive Programming

In 1968, in a presentation that would later become known as the the mother of all demos, computer scientist Douglas Engelbart and his team started the personal computer revolution by demonstrating a system that featured text editing on a screen, his newly invented mouse, mixing of text and graphics, outline views, hypertext links, screen-sharing, and even videoconferencing. At a time when computers were room-sized machines conceived to outperform humans at computational tasks, he instead proposed that they help the human perform intellectual tasks, augmenting human intelligence by becoming interactive assistants in everyones daily work. The graphical user interface was born.

But in addition to its groundbreaking interactivity, Engelbarts system is also interesting for the way it was built: it was written in several different programming languages that were specifically designed for it and adapted as the system changed. Building a truly innovative system also required building appropriate languages to program it. The next important milestone inspired by Engelbarts vision was the Xerox Alto system in 1973. In addition to introducing the desktop metaphor and other user interface innovations, it also featured the first object-oriented language, Smalltalk.

Today, building graphical user interfaces and using object-oriented languages have become mainstream. Unfortunately, though, programming user interfaces is still surprisingly difficult. Code written in the currently predominant style, event-driven programming and the observer pattern, has an uncanny tendency to quickly evolve into an unmaintainable mess, commonly referred to as spaghetti code. Is there a better way?

I think its time for another step in the evolution of user interfaces and programming languages. In recent years, the ideas of functional programming and a (separate) programming style called functional reactive programming (FRP) have shown great promise in making it easier to develop any kind of interactive programs.

This text is one of the first comprehensive introductions to functional reactive programming in book form. With great enthusiasm, Stephen Blackheath and Anthony Jones teach you the basic concepts of FRP, explain a large example in detail, and discuss various patterns that commonly occur in practice. To show that FRP does indeed make things simpler, the pair of brothers also presents an illuminating case study where they solve one problem in three different programming styles and compare the results. Of the three approachesevent-based programming, actors, and functional reactive programmingthe latter compares most favorably.

Functional reactive programming is a style that is usually supported by a library for a particular programming language. The authors have written an FRP library called Sodium that is available for several languages, including Java, and this book profits from their experience in detail. For the sake of concreteness, they use it in this book as well. Of course, the concepts apply more generally, and the authors also present a short guide to many other FRP libraries.

Not all programming languages are created equal. As the name suggests, functional reactive programming derives much of its expressive power from functional programming. In this book, Stephen and Anthony dont assume any prior knowledge of functional programming; instead, they gently introduce you to the necessary concepts as needed. But this heritage also means that FRP libraries can only exist in languages that support them. Java is a popular example, and the authors have chosen it as the main vehicle for explaining FRP. Still, Java is mainly an imperative language, and I think the mismatch is showing in some places. That is why personally, I prefer the purely functional language Haskell for my FRP work. But I think the authors made an excellent choice by picking a more popular and widely used language for this book and not shying away from the difficulties of dealing with the imperative aspects of Java.

Not all FRP libraries are created equal, either. As already mentioned, this book focuses on the Sodium library. But for the case where your programming environment is limited, the authors also discuss libraries like RxJS, which implement a style called reactive programming. This is very similar to functional reactive programming, but the authors rightfully note that it lacks some benefits and guarantees, such as a deterministic merge primitive. I wholeheartedly recommend the Sodium library as designed by the authors.

The field of functional reactive programming is still very young, and you may find that youll need to think in new ways to express your code in this style. This book gives you the necessary tools and foundation for doing that.

H EINRICH A PFELMUS

O PEN SOURCE DEVELOPER

A UTHOR OF THE FRP LIBRARY R EACTIVE-BANANA